Most electric motors have traditionally been provided with both stator and rotor windings, even though in many induction motors the latter may be simplified to a "squirrel cage", and rely upon either conduction through commutators or slip rings, or upon induction, to energize the rotor. Induction motors normally require an alternating supply for their operation, and are not in general well adapted to variable speed operation since their optimum operating speed is ultimately related to the velocityof the rotating field generated by the alternating supply. Direct current motors on the other hand require some form of commutative switching of the supply to the rotor to provide continuous rotation, and such commutators are expensive to build and maintain, as well as a source of undesirable broadband electrical interference. Control of such motors where accurate speeds or displacement control is required remains complex and difficult.
As a result, attention has been given, for a wide range of potential applications ranging from motors for consumer electronic equipment to large applicance, traction and industrial motors, to motors of the reluctance type in which the current through stator windings is switched, usually in modern designs by sold state devices, so as to produce a changing electromagnetic field which will result in progressive angular movement of poles of a stator as it seeks a minimum reluctance position within the field. This movement may be in the form of discrete steps, individually controlled, as in a stepper motor, or the movement of the rotor may be sensed by some suitable means to switch the current through the stator windings so as to provide a free running mode in which successive steps or impulses run together to provide continuous rotation. In an article entitled "Switched Reluctance Motor Drive Systems", published in Design Engineering, May 1984, pages 74-75, such a reluctance motor is described, and the advantages of such motors in variable speed drives are discussed.
Regardless of the mode of operation, the inductance of the windings provides difficulties as they are progressively switched, since it limits the rate of increase of the current upon energization and the rate at which magnetic energy can be dispersed when no longer required, particularly if excessive potentials are not to be induced in the windings.
Various prior art approaches to this problem are discussed in my U.S. Pat. No. 4,584,506, incorporated herein by reference, which discloses an electric motor having a stator with multiple sequentially energizable phase windings and a rotor magnetized to seek a minimum reluctance position within a progressively moving electromagnetic field produced by said phase windings, first controlled switching means in series relative to a D.C. power supply with each phase winding, and means to control said first switching means to produce said progressively moving electromagnetic field, wherein (a) a charge storage capacitor is provided for each such phase winding, with one terminal of said capacitor connected by a low impedance path to said supply, and the other terminal having first and second connections establishing alternative low impedance paths to opposite ends of the winding, the first such connection being established by first diode means to that end of the winding connection to the first switching means, the first diode means being oriented to permit low impedance passage to said capacitor of forward current continuing in said winding after turnoff of the switching means, and the second such connection being established by second controlled switching means, (b) means are provided to run on said second switching means substantially simultaneously with said first switching means to provide low impedance passage of current from said capacitor to said end of the winding remote from the first switching means, and (c) second diode means are provided between the supply and said remote end of the winding such as to present a low impedance path for forward current from the supply, but a high impedance to reverse current.
U. S. Pat. No. 3,534,204, issued Oct. 13, 1970 to Groezinger, discloses an alternator in which two rotor discs having respectively multiple north and south hompoles flank a multipolar annular stator having plural pairs of poles directed towards the poles of the rotor discs with a winding portion around the stator between each pair of poles. The north and south homopoles are staggered so that any particular portion of the stator winding is subjected to alternating magnetic fields as the homopoles of the rotor discs pass that portion. By arranging the rotor homopoles so as to have a width which is a multiple of that of the stator poles, and organizing the winding appropriately, a multiple phase output may be obtained.